This invention relates to a control apparatus for a vehicle having an electric motor for driving the wheels of the vehicle and an electric power supply for energizing the electric motor, and more particularly to the control of the motor field current of the electric motor.
Japanese patent documents such as JP-A-2005-186756, Japanese Patent No. 3610972, and Japanese Patent No. 3594024, disclose conventional procedures for controlling the motor field current in an electric motor, the procedure being used for a control apparatus comprising an electric motor for driving the wheels of a vehicle, a motor field current control unit for controlling the motor field current in the motor, an electric power supply for energizing the electric motor, and a power control unit for controlling the power supplied to the electric motor.
The document JP-A-2005-186756 discloses a procedure wherein the back electromotive force induced in the electric motor is suppressed to yield a sufficient torque at high revolution speed operations through the “weak field control” that the field current of the motor is gradually decreased as the revolution speed of the motor increases exceeding a predetermined threshold value.
The Japanese Patent No. 3610972 teaches a procedure wherein the motor field current in the motor is decreasingly corrected so as to decrease the back electromotive force generated in the motor when the generator is deemed to generate insufficient power due to the change in transmission gear ratio, so that power generation in the generator is recovered and the required torque can be generated.
According to the document Japanese Patent No. 3594024, the motor field current in the motor is decreased in accordance with the decrease in the motor torque; the rate of decreasing the motor field current is increased for smaller limit of power generation; and therefore the back electromotive force induced in the motor is suppressed so that an effective voltage can be obtained to produce current for obtaining a target motor torque.